Prohibit accessing nested classes/objects of class object using class literal

The fqname of class should be clear from code
Example: can't shorten A.Default.B.Default.C to A.B.C
Also fixes problem when nested class of enum class could be accessed via enum entry

compiler/testData/diagnostics/tests/classObjects/nestedClassInPrivateClassObject.kt

@@ -10,6 +10,6 @@ class A {
     }
 }
 
-fun f1() = A.B.<!INVISIBLE_MEMBER!>C<!>
+fun f1() = A.Default.B.<!INVISIBLE_MEMBER!>C<!>
 
-fun f2() = A.B.C.<!INVISIBLE_MEMBER!>foo<!>()
+fun f2() = A.Default.B.C.<!INVISIBLE_MEMBER!>foo<!>()